1. field of the Invention
The present invention relates to combined cycle electric power plants and more particularly to improved heat recovery steam generator control especially useful in the operation of steam generators in combined cycle electric power plants.
2. Description of the Prior Art
In the design of modern electric power plants, it is a significant object to achieve the greatest efficiency possible in the generation of electricity. To this end, steam generators are designed to extract heat sufficiently from and to use heat to convert a fluid such as water into superheated steam at a relatively high pressure. Further, such steam generators have been incorporated into combined cycle electric generating plants including both gas and steam turbines wherein the exhaust gases of the gas turbine are used to heat water into steam to then be transferred to the steam turbine.
Typically, steam generators include a water heating section or economizer tube, an evaporator tube and finally a superheater tube whereby water is gradually heated while increasing levels of pressure are applied thereto to provide from the superheater tube superheated steam to be supplied to the steam turbine. A condenser is associated with the steam turbine to receive the spent steam and for converting it into water condensate to be fed back to the steam generator. In particular, a feedwater pump supplies the condensate through the economizer, evaporator and superheater tubes, wherein the condensate is sequentially heated and additional pressure applied thereto. In the operation of steam generators, it is necessary to maintain a minimum rate of circulation of water or other transfer fluid in the heating tube while heat is being applied thereto in order to prevent the burning of the tubes forming the walls of the furnace. If a minimum flow rate is not established, hot spots may develop within certain portions of the tubes whereby steam is generated locally at these spots thereby tending to block the flow of fluid therethrough with the eventual stagnation of the tube and possible significant damage thereto.
In a combined cycle electric power plant, a steam turbine is combined with a gas turbine whereby the heated exhaust gases of the turbine, otherwise lost to the atmosphere, are used to heat the circulated fluid and to convert it into steam to drive the steam turbine. In this manner, a significant reduction in the fuel required to heat the steam is achieved and the heat contained in the gas turbine exhaust gases is effectively utilized, because flow through the economizer tube is high at low steam rates. Typically in such combined cycle electric power plants, a deaerator is incorporated in the return conduit from the condenser to the steam generator for removing air from the feedwater and also to provide initial heating to the feedwater before it is supplied to the economizer tube. Heat may be provided from the steam turbine itself whereby steam is extracted between selected stages of the steam turbine and is selectively fed to the deaerator, to be sprayed therein whereby the returned condensate is exposed to the sprayed steam, thus raising the temperature of the condensate. However, extraction steam is available from the steam turbine only when it is being operated in the range of 85-95% of full load. Thus, at relatively low loads, the feedwater entering the deaerator is not heated by extraction steam, but is heated at a relatively low load by the economizer tube.
Further, at relatively low loads as imposed upon the combined cycle electric power plant, it is not necessary to extract significant amounts of heat from the exhaust gases of the gas turbine, the heat therein being dissipated into the atmosphere through a louvre at the top of the steam generator. In the prior art steam generators, significant heat is lost, especially at relatively low operating loads.
Steam generators are known, for example that described in U.S. Pat. No. 3,038,453, of the force flow once-through type in which a portion of the transfer fluid is recirculated. In particular, recirculation paths are provided about each section or portion of the steam generator, i.e., the economizer, evaporator and superheater tubes. Thus, at relatively low steam turbine load, e.g., below 30% of full load, an additional amount of the working fluid is recirculated through the heating tubes whereby a minimum flow rate is established to prevent possible damage, e.g., burnout, of these tubes.
The description of prior art herein is made on good faith and no representation is made that any prior art considered is the best pertaining prior art nor that the interpretation placed on it is unrebuttable.